Field of the Invention
The present invention relates to a dither mask generation method and device, and in particular relates to a technology of generating a dither mask used in halftone processing utilized in ink ejection control of an inkjet printer.
Description of the Related Art
Japanese Patent Application Laid-Open No. 2010-162770 gazette discloses a serial type inkjet printer that controls ejection of each nozzle of a print head by performing halftone processing by an ordered dither method using a dither mask. The serial type inkjet printer records images on a recording medium by repeating a scan operation of ejecting ink from a nozzle of a print head while moving the print head along a main scanning direction and a medium conveying operation which is sub scanning feed of intermittently conveying the recording medium in a sub scanning direction.
In such a recording method, behavior of impacted droplets on a recording medium, that is, dot behavior, is changed by recording position errors of individual dots to the recording medium by individual nozzles of a print head, ejection amount errors of the individual nozzles, a recording order or recording timing of dots or the like. Due to the change of the dot behavior on the recording medium, density irregularity called “banding” in which a recording density changes in a repetition cycle of individual print paths, a boundary of the individual print paths is conspicuous or the like is generated. When banding is generated, a problem that print image quality declines arises.
For such a problem, in the printer described in Japanese Patent Application Laid-Open No. 2010-162770 gazette, banding is suppressed by setting a threshold of the dither mask such that a nozzle using rate of at least one of nozzle groups (including most distal end nozzles arranged at both individual ends) arranged at each of both ends of a nozzle array of the print head becomes lower than a nozzle using rate of an intermediate nozzle group arranged between the nozzle groups at both ends. Japanese Patent Application Laid-Open No. 2010-162770 gazette discloses a method of generating a dither mask that makes a dot density of pixels recorded by the nozzle groups at both ends of the nozzle array lower than a dot density of pixels recorded by the intermediate nozzle group to suppress banding.
It is understood that “print head” in Japanese Patent Application Laid-Open No. 2010-162770 gazette is a term corresponding to “recording head” in the present specification. Also, it is understood that “nozzle using rate” in Japanese Patent Application Laid-Open No. 2010-162770 gazette is a term corresponding to “nozzle ejection rate” in the present specification.